This year will be among the three hottest on record, the United Nations says, as almost 200 countries begin talks in Germany to bolster a global climate accord that the United States plans to quit.

Temperatures this year will be slightly less than during a record-breaking 2016 and roughly level with 2015, the United Nations' World Meteorological Organisation (WMO) said, part of a long-term warming trend driven by greenhouse gases from burning fossil fuels.

"We have witnessed extraordinary weather," WMO head Petteri Talaas said, pointing to extreme events including a spate of hurricanes in the Atlantic and Caribbean, monsoon floods in Asia and drought in East Africa.

The WMO attributed the small dip from last year to the fading effects of a natural El Nino event that released extra heat from the Pacific Ocean in 2016.

In terms of economic costs, 2017 will be the most costly hurricane season on record after Harvey, Irma and Maria, it added.